CARBONACEOUS MATERIAL-COATED GRAPHITE PARTICLES, NEGATIVE ELECTRODE FOR LITHIUM ION SECONDARY BATTERIES, LITHIUM ION SECONDARY BATTERY, AND METHOD FOR PRODUCING CARBONACEOUS MATERIAL-COATED GRAPHITE PARTICLES

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2024/007319, filed Feb. 28, 2024, which claims priority to Japanese Patent Application No. 2023-033770, filed Mar. 6, 2023, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to carbonaceous substance-coated graphite particles, a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method of producing carbonaceous substance-coated graphite particles.

BACKGROUND OF THE INVENTION

A lithium ion secondary battery includes a negative electrode, a positive electrode, and a non-aqueous electrolyte as main components. Lithium ions move between the negative electrode and the positive electrode in the discharging and charging processes, thereby causing a secondary battery to operate.

Conventionally, graphite particles obtained by spherization (spherically-shaped graphite) have been used as a negative electrode material for a lithium ion secondary battery in some cases (Patent Literature 1).

Patent Literature

Patent Literature 1: JP 2014-146607 A

SUMMARY OF THE INVENTION

When conventional graphite particles (e.g., spherically-shaped graphite) are used as a negative electrode material for a lithium ion secondary battery, battery characteristics such as a cycle capacity maintaining characteristic are sometimes insufficient.

The present inventors have made a study and as a result found that the use of carbonaceous substance-coated graphite particles in which surfaces of graphite particles are coated with a carbonaceous substance can improve the battery characteristics.

Accordingly, aspects of the present invention have an object to provide carbonaceous substance-coated graphite particles that exhibit an excellent cycle capacity maintaining characteristic when used as a negative electrode material for a lithium ion secondary battery.

The inventors found, though an earnest study, that a carbonaceous substance having specific crystallinity can be obtained through optimization of baking conditions in manufacture of carbonaceous substance-coated graphite particles, and as a result the foregoing object can be achieved. Thus, aspects of the invention have been completed.

Specifically, aspects of the present invention include the following [1] to [7].

• • [1] Carbonaceous substance-coated graphite particles comprising:
  • graphite particles; and
  • a carbonaceous substance covering at least part of surfaces of the graphite particles,
  • wherein an elastic modulus determined using a scanning probe microscope is not less than 10 GPa.
  • [2] The carbonaceous substance-coated graphite particles according to [1] above,
  • wherein a particle size is 5.0 to 15.0 μm, and a specific surface area is 3.0 to 15.0 m²/g.
  • [3] The carbonaceous substance-coated graphite particles according to [1] or [2] above,
  • wherein a content of the carbonaceous substance is 1.0 to 30.0 mass % with respect to a total mass of the carbonaceous substance-coated graphite particles.
  • [4] The carbonaceous substance-coated graphite particles according to any one of [1] to [3] above,
  • wherein the carbonaceous substance-coated graphite particles are used as a negative electrode material for a lithium ion secondary battery.
  • [5] A negative electrode for a lithium ion secondary battery, the negative electrode comprising the carbonaceous substance-coated graphite particles according to any one of [1] to [3] above.
  • [6] A lithium ion secondary battery comprising the negative electrode according to [5] above.
  • [7] A method of producing the carbonaceous substance-coated graphite particles according to any one of [1] to [3] above, the method comprising:
  • attaching a carbonaceous precursor that is a precursor of the carbonaceous substance to the graphite particles to thereby obtain precursor-attached graphite particles,
  • performing first baking to heat the precursor-attached graphite particles at a temperature of higher than 900° C. and not higher than 1,500° C. in a non-oxidizing atmosphere,
  • after the first baking, performing intermediate exposure to expose the precursor-attached graphite particles to an oxidizing atmosphere at a temperature of 50° C. to 100° C., and
  • after the intermediate exposure, performing second baking to heat the precursor-attached graphite particles at a temperature of 1,100° C. to 1,500° C. in a non-oxidizing atmosphere.

Aspects of the present invention make it possible to provide carbonaceous substance-coated graphite particles that exhibit an excellent cycle capacity maintaining characteristic when used as a negative electrode material for a lithium ion secondary battery.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a cross-sectional view of a battery for evaluation prepared for battery characteristic evaluation in Examples and Comparative Examples.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the description, a range expressed using the form of “(numeral) to (another numeral)” should read as a range including both ends defined by the numerals. For example, a range expressed as “A to B” includes A and B.

Carbonaceous Substance-Coated Graphite Particles

Carbonaceous substance-coated graphite particles of the present embodiment (hereinafter also referred to as “present carbonaceous substance-coated graphite particles”) comprise: graphite particles; and a carbonaceous substance covering at least part of surfaces of the graphite particles, wherein an elastic modulus determined using a scanning probe microscope is not less than 10 GPa.

When the present carbonaceous substance-coated graphite particles are used as a negative electrode material for a lithium ion secondary battery, the cycle capacity maintaining characteristic is excellent.

Elastic Modulus

With respect to the present carbonaceous substance-coated graphite particles, the elastic modulus determined using a scanning probe microscope (SPM) (hereinafter also simply referred to as “elastic modulus”) is not less than 10 GPa, more preferably not less than 12 GPa, still more preferably not less than 14 GPa, and particularly preferably not less than 16 GPa for the reason of the cycle capacity maintaining characteristic being excellent.

Although the reason of the cycle capacity maintaining characteristic being excellent is not clear, this is presumably because a carbonaceous substance sufficiently covers the surfaces of graphite particles when the elastic modulus is within the foregoing range, while a carbonaceous substance covering the surfaces of graphite particles is insufficient when the elastic modulus is less than the foregoing range.

At the same time, the elastic modulus of the present carbonaceous substance-coated graphite particles is preferably not more than 25 GPa, more preferably not more than 24 GPa, and still more preferably not more than 22 GPa for the reason of excellent fast charging characteristic.

The measurement of elastic modulus using a scanning probe microscope (SPM) is carried out as follows.

First, epoxy resin is thinly applied onto a silicon substrate, a sample (carbonaceous substance-coated graphite particles) is sprayed onto the epoxy resin, and then the silicon substrate is fixed. Next, a probe is moved along a particle surface to scan the surface under the conditions below (with an indentation load of 200 nN for each particle), thereby obtaining a force curve. The DMT model is fitted to the obtained force curve. Thus, the elastic modulus (unit: GPa) of each particle is determined. The average of the determined elastic moduli is obtained as the elastic modulus of the sample (carbonaceous substance-coated graphite particles).

• • SPM: Dimension ICON (manufactured by Bruker)
  • SPM controller: NanoScope V (manufactured by Bruker)
  • Probe: silicon cantilever (manufactured by Bruker)
  • Specifications of probe (cantilever): made of single-crystal Si; radius of curvature of 8 nm
  • Measurement region: 2 μm×2 μm
  • Number of measurements: 512×512 points
  • Measurement mode: QNM Scan mode (mode for measuring the shape and the elastic modulus)
  • Measurement atmosphere: argon atmosphere

Carbonaceous Substance Content

The carbonaceous substance content is preferably not less than 1.0 mass %, more preferably not less than 3.0 mass %, still more preferably not less than 3.5 mass %, and particularly preferably not less than 4.5 mass % with respect to the total mass of carbonaceous substance-coated graphite particles.

With the carbonaceous substance content being within this range, active edge surfaces of graphite particles are easily covered, resulting in an excellent initial charging-discharging characteristic.

At the same time, the carbonaceous substance content is preferably not more than 30.0 mass %, more preferably not more than 25.0 mass %, still more preferably not more than 20.0 mass %, yet still more preferably not more than 15.0 mass %, particularly preferably not more than 10.0 mass %, and most preferably not more than 8.0 mass %.

With the carbonaceous substance content being within this range, the amount of carbonaceous substance with relatively low discharging capacity is to be small, resulting in excellent discharging capacity.

In addition, with the carbonaceous substance content being within this range, the amount of used carbonaceous precursor described later is to be small, and this leads to less occurrence of fusion during mixing and baking described later and suppresses breakage and peeling of the finally obtained carbonaceous substance, resulting in an excellent initial charging-discharging characteristic.

It suffices if the average of carbonaceous substance contents of all carbonaceous substance-coated graphite particles falls within the foregoing range. Not every carbonaceous substance-coated graphite particle needs to have the carbonaceous substance content within the foregoing range, and carbonaceous substance-coated graphite particles having the carbonaceous substance content outside the foregoing range may be partly included.

The carbonaceous substance content is determined from the amount of residual carbon in a carbonaceous precursor which has been alone baked under the same conditions as those for baking of precursor-attached graphite particles to be described later.

For convenience, the carbonaceous substance content with respect to the total mass of carbonaceous substance-coated graphite particles is hereinafter sometimes simply referred to as “carbonaceous substance content.”

Particle Size

The particle size of the present carbonaceous substance-coated graphite particles is preferably not less than 5.0 μm, more preferably not less than 6.0 μm, and still more preferably not less than 6.5 μm.

At the same time, the particle size of the present carbonaceous substance-coated graphite particles is preferably not more than 15.0 μm, more preferably not more than 12.0 μm, still more preferably not more than 10.0 μm, and particularly preferably not more than 9.0 μm.

The particle size is a median diameter at which a cumulative frequency of a particle size distribution obtained with a laser diffraction type particle size distribution analyzer (LMS2000e, manufactured by Seishin Enterprise Co., Ltd.) is 50% by volume.

Specific Surface Area

The specific surface area of the present carbonaceous substance-coated graphite particles is preferably not less than 3.0 m²/g, more preferably not less than 4.0 m²/g, still more preferably not less than 4.5 m²/g, and particularly preferably not less than 4.0 m²/g.

At the same time, the specific surface area of the present carbonaceous substance-coated graphite particles is preferably not more than 20.0 m²/g, more preferably not more than 18.0 m²/g, still more preferably not more than 15.0 m²/g, and particularly preferably not more than 14.5 m²/g.

The specific surface area is determined by the BET method, more specifically, determined through nitrogen gas adsorption according to JIS Z 8830:2013 (determination of the specific surface area of powders (solids) by gas adsorption).

Method of Producing Carbonaceous Substance-Coated Graphite Particles

Next, a method of producing carbonaceous substance-coated graphite particles in the present embodiment (hereinafter also referred to as “present production method”) is described.

The present production method is a method of producing the present carbonaceous substance-coated graphite particles described above. In summary, a carbonaceous precursor is attached to graphite particles, whereafter first baking, intermediate exposure, and second baking (hereinafter also simply collectively called “baking”) to be described later are performed in this order.

In the following, graphite particles and a carbonaceous precursor used in the present production method are first described, and then, respective steps in the present production method are described.

Graphite Particles

A method of producing graphite particles used in the present production method is not particularly limited, and one example is processing a raw material into a spherical shape.

The raw material here is graphite having a shape other than a spherical shape (including an ellipsoidal shape), such as scale-like graphite. The graphite may be either natural graphite or artificial graphite, while natural graphite is preferred because of high crystallinity or other reasons.

Specific examples of the method of processing the raw material into a spherical shape (the method of shaping the raw material into a spherical shape) include a method in which the raw material is stirred in the presence of a granulation aid such as an adhesive or a resin, a method in which mechanical external force is applied to the raw material without use of a granulation aid, and a method in which both methods are combined.

Among these, the method in which mechanical external force is applied to the raw material without use of a granulation aid is preferred. This method is described below in more detail.

More specifically, mechanical external force is applied to the raw material (e.g., scale-like graphite) with a pulverizing apparatus to pulverize and granulate the raw material. The raw material is thus shaped into a spherical shape to obtain spherically-shaped graphite.

Examples of the pulverizing apparatus include a rotating ball mill, Counter Jet Mill (manufactured by Hosokawa Micron Corporation), Current Jet (manufactured by Nisshin Engineering Inc.), Hybridization System (manufactured by Nara Machinery Co., Ltd.), CF mill (manufactured by UBE Corporation), MECHANO FUSION System (manufactured by Hosokawa Micron Corporation), and Theta Composer (manufactured by Tokuju Corporation), and among these, Hybridization System (manufactured by Nara Machinery Co., Ltd.) is preferred.

In the present production method, it is preferable that a plurality of pulverizing apparatuses be arranged in series and the raw material sequentially pass through the pulverizing apparatuses. In other words, the pulverizing apparatuses are preferably arranged in series such that immediately after passing through one pulverizing apparatus, the raw material is pulverized and granulated in the next pulverizing apparatus.

Here, the number of pulverizing apparatuses is, for example, 2 or more, preferably 3 or more, more preferably 4 or more, still more preferably 5 or more, and particularly preferably 6 or more.

At the same time, the number of pulverizing apparatuses is preferably 10 or less, more preferably 8 or less, and still more preferably 7 or less.

A time for pulverizing and granulating the raw material (hereinafter also referred to as “pulverizing time”) in each pulverizing apparatus is preferably not less than 8 minutes, more preferably not less than 13 minutes, and still more preferably not less than 18 minutes.

At the same time, the pulverizing time in each pulverizing apparatus is preferably not more than 60 minutes, more preferably not more than 50 minutes, and still more preferably not more than 40 minutes.

A product of the number of pulverizing apparatuses and the pulverizing time in each pulverizing apparatus (hereinafter also referred to as “total pulverizing time”) is preferably not less than 30 minutes, more preferably not less than 50 minutes, and still more preferably not less than 90 minutes.

At the same time, the total pulverizing time is preferably not more than 180 minutes, and more preferably not more than 160 minutes.

The pulverizing apparatus usually includes a rotor built therein.

The rotor in each pulverizing apparatus has a circumferential speed of preferably not less than 30 m/second, more preferably not less than 40 m/second, and still more preferably not less than 60 m/second.

At the same time, the circumferential speed of the rotor in each pulverizing apparatus is preferably not more than 100 m/second, and more preferably not more than 80 m/second.

The amount of the raw material charged into each pulverizing apparatus is preferably small for easier application of sheer force and compression force to the raw material.

Carbonaceous Precursor

The carbonaceous precursor is a precursor of the above-mentioned carbonaceous substance and becomes the carbonaceous substance covering the graphite particles by carbonization through baking to be described later.

The carbonaceous precursor is exemplified by resins being carbon materials that have low crystallinity compared to graphite and thus would never form a graphite crystal even when subjected to high temperature treatment necessary for graphitization.

Examples of resins include polyvinyl alcohol, polyacrylic acid, phenolic resin, furan resin, cellulose resin, polystyrene resin, polyimide resin, and epoxy resin.

Phenolic resin is preferred in terms of battery characteristics of the resulting battery.

The carbonaceous precursor is preferably in a powder form for easier spread over surfaces of the graphite particles. The median diameter (D₅₀) of the carbonaceous precursor in a powder form is not particularly limited and is, for example, 1 to 50 μm.

Preparation of Precursor-Attached Graphite Particles

In the present production method, first, the carbonaceous precursor is attached to the graphite particles. By this process, precursor-attached graphite particles in which the carbonaceous precursor is attached to at least part of surfaces of the graphite particles are obtained.

An example of a method of attaching the carbonaceous precursor to the graphite particles is a method involving mixing the graphite particles with the carbonaceous precursor.

The mixing method is not particularly limited, and an example thereof is a method in which the graphite particles and the carbonaceous precursor being in a powder form or having been heated and melted into a liquid form are mixed using a kneader or another apparatus. In this process, a dispersion liquid in which the graphite particles are dispersed in a dispersion medium may be used. As a kneader, a pressure kneader or a two-roll mill may be used, for example.

Amount of Carbonaceous Precursor Added

The amount of the carbonaceous precursor added (the amount of the carbonaceous precursor to be attached to the graphite particles) is suitably adjusted in accordance with a desired amount of a carbonaceous substance to be finally obtained.

Baking

Next, the obtained precursor-attached graphite particles are subjected to baking (first baking, intermediate exposure, and second baking). By this process, the carbonaceous precursor is carbonized to turn into a carbonaceous substance.

Thus, carbonaceous substance-coated graphite particles in which at least part of surfaces of the graphite particles is coated with the carbonaceous substance are obtained. The obtained carbonaceous substance-coated graphite particles have an elastic modulus satisfying the foregoing range.

First Baking

In the first baking, the precursor-attached graphite particles are heated at a temperature of higher than 900° C. and not higher than 1,500° C. (hereinafter also referred to as “first baking temperature”) in a non-oxidizing atmosphere.

The reason of the atmosphere for the first baking being a non-oxidizing atmosphere is that the carbonaceous substance would be burned and disappear in an oxidizing atmosphere.

Examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon atmosphere, a helium atmosphere, and a vacuum atmosphere. Alternatively, coke breeze which oxidizes by itself may be disposed to decrease an oxygen concentration in the atmosphere to thereby prepare a substantially non-oxidizing atmosphere.

The first baking temperature is higher than 900° C., preferably not lower than 950° C., and more preferably not lower than 1,000° C. At the same time, the first baking temperature is not higher than 1,500° C., preferably not higher than 1,400° C., and more preferably not higher than 1,250° C.

A time of performing the first baking (a time for which the precursor-attached graphite particles are heated at the first baking temperature in the non-oxidizing atmosphere) is preferably not less than 0.5 hours, more preferably not less than 1 hour, and still more preferably not less than 2 hours. The upper limit thereof is not particularly limited and is for instance 30 hours, preferably 10 hours, and more preferably 5 hours.

The precursor-attached graphite particles having undergone the first baking is preferably cooled to an intermediate exposure temperature (described later) with the atmosphere for the first baking (non-oxidizing atmosphere) being kept.

Intermediate Exposure

In the intermediate exposure, the precursor-attached graphite particles having undergone the first baking (and then having been cooled) are exposed to an oxidizing atmosphere at a temperature of 50° C. to 100° C. (hereinafter also called “intermediate exposure temperature”).

Examples of the oxidizing atmosphere include an atmosphere containing oxygen, ozone, carbon dioxide, and the like, and the air atmosphere is preferred for the sake of convenience.

The intermediate exposure temperature is not lower than 50° C., preferably not lower than 55° C., and more preferably not lower than 60° C. At the same time, the intermediate exposure temperature is not higher than 100° C., preferably not higher than 90° C., and more preferably not higher than 80° C.

A time of performing the intermediate exposure (a time for which the precursor-attached graphite particles are exposed to the oxidizing atmosphere at the intermediate exposure temperature) is preferably not less than 10 minutes, more preferably not less than 20 minutes, and still more preferably not less than 30 minutes. The upper limit thereof is not particularly limited and is for instance 120 minutes, preferably 90 minutes, and more preferably 60 minutes.

Second Baking

In the second baking, the precursor-attached graphite particles having undergone the intermediate exposure are heated at a temperature of 1,100° C. to 1,500° C. (hereinafter also called “second baking temperature”) in a non-oxidizing atmosphere.

The reason of the atmosphere for the second baking being a non-oxidizing atmosphere and preferred examples of the non-oxidizing atmosphere are the same as those for the first baking.

The second baking temperature is not lower than 1,100° C., preferably not lower than 1,150° C., and more preferably not lower than 1,200° C. The second baking temperature is preferably not lower than the first baking temperature.

Meanwhile, the second baking temperature is not higher than 1,500° C., preferably not higher than 1,400° C., and more preferably not higher than 1,300° C.

A time of performing the second baking (a time for which the precursor-attached graphite particles are heated at the second baking temperature in the non-oxidizing atmosphere) is preferably not less than 0.5 hours, more preferably not less than 1 hour, and still more preferably not less than 2 hours. The upper limit thereof is not particularly limited and is for instance 30 hours, preferably 10 hours, and preferably 5 hours.

Negative Electrode for Lithium Ion Secondary Battery (Negative Electrode)

A negative electrode for a lithium ion secondary battery in the present embodiment contains the present carbonaceous substance-coated graphite particles described above. The “carbonaceous substance-coated graphite particles” are hereinafter sometimes referred to as “negative electrode material.” Besides, the negative electrode for a lithium ion secondary battery is also simply referred to as “negative electrode.”

The negative electrode of the present embodiment is prepared as with a general negative electrode.

For preparation of the negative electrode, it is preferable to use a negative electrode mixture preliminarily prepared by adding a binder to the negative electrode material. The negative electrode mixture may contain an active material or an electrically conductive material in addition to the negative electrode material.

For the binder, a binder that is chemically and electrochemically stable against an electrolyte is favorable, and use may be made of, for example, fluororesin such as polytetrafluoroethylene or polyvinylidene fluoride, resin such as polyethylene, polyvinyl alcohol, or styrene butadiene rubber, and carboxymethyl cellulose, while two or more of these can be used in combination.

The binder normally accounts for about 1 to 20 mass % of the total amount of the negative electrode mixture.

More specifically, first, the negative electrode material is optionally adjusted to a desired particle size through classification or the like. Thereafter, the negative electrode material is mixed with the binder, and the resulting mixture is dispersed in a solvent to prepare a negative electrode mixture in a paste form. Examples of the solvent include water, isopropyl alcohol, N-methylpyrrolidone, and dimethylformamide. In the mixing and dispersing processes, a known agitator, mixer, kneader, or the like is used.

The prepared paste is applied on one or both of the surfaces of a current collector and dried. The coating weight is preferably 3 to 15 mg/cm² and more preferably 5 to 15 mg/cm². This process results in a negative electrode mixture layer (negative electrode) that is uniformly and firmly attached to the current collector. The negative electrode mixture layer has a thickness of preferably 10 to 200 μm and more preferably 20 to 100 μm.

After the negative electrode mixture layer is formed, compression bonding such as press pressurization is performed, whereby the adhesion strength of the negative electrode mixture layer (negative electrode) to the current collector can be further improved.

The shape of the current collector is not particularly limited, and examples thereof include a foil-like shape, a mesh shape, and a net-like shape such as an expanded metal shape. The material of the current collector is preferably copper, stainless steel, nickel or the like. The current collector preferably has a thickness of about 5 to 20 μm in a case of a foil-like shape.

Coated Electrode Density

The negative electrode of the present embodiment has a coated electrode density of preferably not less than 1.10 g/cm³ and more preferably not less than 1.20 g/cm³, and at the same time, preferably not more than 2.00 g/cm³ and more preferably not more than 1.90 g/cm³.

The coated electrode density of the negative electrode is determined as described below.

A negative electrode punched out to a given area is measured for mass (with use of an electronic balance) and thickness (with use of a micrometer). Subsequently, 10 pieces of the current collector punched out to the same area are measured for mass, and the average value thereof is specified as the mass of the current collector. Further, the thickness of the current collector is determined from the density of metal constituting the current collector. Then, the coated electrode density of the negative electrode is determined by the following equation.

Coated electrode density of negative electrode=(Mass of negative electrode−Mass of current collector)/(Thickness of negative electrode−Thickness of current collector)×(Punched out area)

Lithium Ion Secondary Battery

A lithium ion secondary battery of the present embodiment includes the negative electrode of the present embodiment and, in addition, components such as a positive electrode and a non-aqueous electrolyte.

The lithium ion secondary battery of the present embodiment is composed of, for example, a negative electrode, a non-aqueous electrolyte, and a positive electrode that are stacked in this order and accommodated in an exterior material.

The type of the lithium ion secondary battery of the present embodiment can be arbitrarily selected from a cylindrical type, a square type, a coin type, a button type, and other types, depending on the intended use, the device to which the battery is to be mounted, the required charging-discharging capacity, or the like.

Positive Electrode

For a material of the positive electrode (positive electrode active material), a material that can absorb and store as well as release lithium in a sufficient amount is preferably selected. Examples of the positive electrode active material include, in addition to lithium, a lithium-containing compound such as a lithium-containing transition metal oxide, a transition metal chalcogenide, or a vanadium oxide or a lithium compound thereof; a Chevrel phase compound expressed by General Formula M_xMo₆S_8−Y (where M represents at least one transition metal element, X is a numerical value in the range of 0≤X≤4, and Y is a numerical value in the range of 0≤Y≤1); activated carbon; and activated carbon fiber. The vanadium oxide is expressed by V₂O₅, V₆O₁₃, V₂O₄ or V₃O₈.

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and lithium and two or more transition metals may be mixed to form a solid solution as the lithium-containing transition metal oxide. A single composite oxide may be used alone, or two or more composite oxides may be used in combination.

The lithium-containing transition metal oxide is specifically expressed by LiM¹_1−XM²_XO₂ (where M¹ and M² represent at least one transition metal element, and X is a numerical value in the range of 0≤X≤1) or LiM¹_1−YM²_YO₄ (where M¹ and M² represent at least one transition metal element, and Y is a numerical value in the range of 0≤Y≤1).

The transition metal element represented by M¹ and M² may be Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, Sn, or the like, with Co, Fe, Mn, Ti, Cr, V, and Al being preferred for instance. Preferred specific examples include LiCoO₂, LiNiO₂, LiMnO₂, LiNi_0.9Co_0.1O₂, and LiNi_0.5Co_0.5O₂.

Using, for example, oxides, hydroxides, and salts of lithium and a transition metal as starting materials, the starting materials are mixed according to the composition of a desired metal oxide and baked in an oxygen atmosphere at a temperature of 600° C. to 1,000° C. to obtain the lithium-containing transition metal oxide.

For the positive electrode active material, any of the foregoing compounds may be used alone, or two or more thereof may be used in combination. For instance, a carbonate such as lithium carbonate can be added to the positive electrode. When the positive electrode is formed, various additives such as an electrically conductive agent and a binder that are conventionally known can be suitably used.

The positive electrode is prepared by, for example, coating both surfaces of a current collector with a positive electrode mixture comprising a positive electrode active material, a binder, and an electrically conductive agent for imparting electrical conductivity to the positive electrode, to thereby form a positive electrode mixture layer.

For the binder, a binder used in preparation of the negative electrode can be used.

For the electrically conductive agent, a conventionally known electrically conductive agent such as a graphitized substance or carbon black is used.

The shape of the current collector is not particularly limited, and examples thereof include a foil-like shape and a net-like shape. The material of the current collector is aluminum, stainless steel, nickel, or the like. The current collector preferably has a thickness of 10 to 40 μm.

As with the negative electrode, the positive electrode may be prepared by applying the positive electrode mixture in a paste form to the current collector, followed by drying and then compression bonding such as press pressurization.

Non-Aqueous Electrolyte

The non-aqueous electrolyte may be a liquid non-aqueous electrolyte (non-aqueous electrolytic solution), or a polyelectrolyte such as a solid electrolyte or a gel electrolyte.

As the non-aqueous electrolyte, use is made of a lithium salt which is an electrolyte salt used for an ordinary non-aqueous electrolytic solution, such as LiPF₆, LiBF₄, LiAsF₆, LiClO₄, LiB(C₆H₅), LiCl, LiBr, LiCF₃SO₃, LiCH₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, LiN(CF₃CH₂OSO₂)₂, LiN(CF₃CF₂OSO₂)₂, LiN(HCF₂CF₂CH₂OSO₂)₂, LiN((CF₃)₂CHOSO₂)₂, LiB[{C₆H₃(CF₃)₂}]₄, LiAlCl₄, or LiSiF₆. From the oxidative stability point of view, LiPF₆ and LiBF₄ are preferred.

The electrolyte salt concentration in the non-aqueous electrolytic solution is preferably 0.1 to 5.0 mol/L and more preferably 0.5 to 3.0 mol/L.

Examples of a solvent used to prepare the non-aqueous electrolytic solution include a carbonate such as ethylene carbonate, propylene carbonate, dimethyl carbonate, or diethyl carbonate; an ether such as 1,1- or 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, 1,3-dioxolan, 4-methyl-1,3-dioxolan, anisole, or diethyl ether; a thioether such as sulfolane or methyl sulfolane; a nitrile such as acetonitrile, chloronitrile, or propionitrile; and an aprotic organic solvent such as trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene, benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethylsulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, or dimethyl sulfite.

When the non-aqueous electrolyte is a polyelectrolyte such as a solid electrolyte or a gel electrolyte, a polymer gelated with a plasticizer (non-aqueous electrolytic solution) is preferably used as a matrix.

For a polymer constituting the matrix, use is suitably made of an ether-based polymer compound such as polyethylene oxide or a crosslinked product thereof; a poly(meth)acrylate-based polymer compound; or a fluorine-based polymer compound such as polyvinylidene fluoride, or vinylidene fluoride-hexafluoropropylene copolymer.

The electrolyte salt concentration in the non-aqueous electrolytic solution that is a plasticizer is preferably 0.1 to 5.0 mol/L and more preferably 0.5 to 2.0 mol/L.

The plasticizer content of the polyelectrolyte is preferably 10 to 90 mass % and more preferably 30 to 80 mass %.

Separator

A separator can also be used in the lithium ion secondary battery of the present embodiment.

The material of the separator is not particularly limited, and use is made of, for example, woven fabric, non-woven fabric, and a fine porous film made of synthetic resin. Among these, a fine porous film made of synthetic resin is preferred, and in particular a polyolefin-based fine porous film is more preferred in terms of the thickness, film strength, and film resistance. Suitable examples of the polyolefin-based fine porous film include a polyethylene fine porous film, a polypropylene fine porous film, and a fine porous film formed by combining those.

EXAMPLES

Aspects of the invention are specifically described below with reference to Examples. However, the invention is not limited to the examples described below.

Example 1

Preparation of Graphite Particles

Scale-like natural graphite (particle size: 8 μm) as the raw material was sequentially passed through 5 pulverizing apparatuses (Hybridization system manufactured by Nara Machinery Co., Ltd.) that were installed in series. In each pulverizing apparatus, the pulverizing time was 30 minutes, and the circumferential speed of the rotor was 50 m/second. The raw material was thus pulverized and granulated to obtain graphite particles being spherically-shaped graphite.

Preparation of Precursor-Attached Graphite Particles

A carbonaceous precursor being pulverized phenolic resin was added to the obtained graphite particles (spherically-shaped graphite) and mixed at room temperature for 30 minutes using a biaxial kneader. The carbonaceous precursor was added in such an amount that a content of the finally obtained carbonaceous substance would be 4.0 mass %. Thus, precursor-attached graphite particles having the carbonaceous precursor (phenolic resin) attached to surfaces of the graphite particles (spherically-shaped graphite) were obtained.

Preparation of Carbonaceous Substance-Coated Graphite Particles

Next, the obtained precursor-attached graphite particles were subjected to baking (first baking, intermediate exposure, and second baking) using a tubular furnace.

Specifically, firstly in the first baking, an atmosphere (non-oxidizing atmosphere) where nitrogen was circulated at 5 L/min was prepared, and then the precursor-attached graphite particles were heated at a first baking temperature of 950° C. for 60 minutes and subsequently cooled to an intermediate exposure temperature to be described below without changing the atmosphere.

Next, in the intermediate exposure, the cooled precursor-attached graphite particles were exposed to an air atmosphere (oxidizing atmosphere) at an intermediate exposure temperature of 80° C. for 30 minutes.

Thereafter, in the second baking, an atmosphere (non-oxidizing atmosphere) where nitrogen was circulated at 2 L/min was prepared, and then the precursor-attached graphite particles were heated at a second baking temperature of 1,100° C. for 60 minutes.

Consequently, the carbonaceous precursor (phenolic resin) turned into a carbonaceous substance, thus obtaining carbonaceous substance-coated graphite particles in which surfaces of the graphite particles (spherically-shaped graphite) were coated with the carbonaceous substance.

Physical properties of the obtained carbonaceous substance-coated graphite particles were determined by the foregoing methods. The results are shown in Table 1 below.

Preparation of Negative Electrode

To water, 98 parts by mass of the carbonaceous substance-coated graphite particles (negative electrode material), 1 part by mass of carboxymethylcellulose (binder), and 1 part by mass of styrene butadiene rubber (binder) were added and stirred to prepare a negative electrode mixture paste.

The prepared negative electrode mixture paste was applied over copper foil (thickness: 16 μm) to have a uniform thickness and then dried in vacuum at 90° C. to form a negative electrode mixture layer. Next, the negative electrode mixture layer was pressurized at a pressure of 120 MPa by hand press. Thereafter, the copper foil and the negative electrode mixture layer were punched out into a circular shape with a diameter of 15.5 mm. A negative electrode (thickness: 60 μm, coated electrode density: 1.20 g/cm³) closely attached to a current collector made of the copper foil was prepared in this manner.

Preparation of Positive Electrode

Lithium metal foil having been pressed against a nickel net was punched out into a circular shape with a diameter of 15.5 mm. Thus, a positive electrode made of the lithium metal foil (thickness: 0.5 mm) closely attached to a current collector made of the nickel net was prepared.

Preparation of Battery for Evaluation

As a battery for evaluation, a button-type secondary battery illustrated in the figure was prepared.

The figure is a cross-sectional view of a button-type secondary battery. In the button-type secondary battery illustrated in the figure, circumferential portions of an exterior cup 1 and an exterior can 3 are crimped with an insulating gasket 6 being interposed therebetween, whereby a tightly sealed structure is formed. In the tightly sealed structure, a current collector 7a, a positive electrode 4, a separator 5, a negative electrode 2, and a current collector 7b are stacked in this order from the inner surface of the exterior can 3 toward the inner surface of the exterior cup 1.

The button-type secondary battery illustrated in the figure was prepared as described below.

First, into a mixed solvent comprising ethylene carbonate (33 vol %) and methylethyl carbonate (67 vol %), LiPF₆ was dissolved at a concentration of 1 mol/L to prepare a non-aqueous electrolytic solution. A polypropylene porous body (thickness: 20 μm) was impregnated with the prepared non-aqueous electrolytic solution to prepare the separator 5 impregnated with the non-aqueous electrolytic solution.

Next, the prepared separator 5 was held between the negative electrode 2 closely attached to the current collector 7b made of copper foil and the positive electrode 4 closely attached to the current collector 7a made of a nickel net, so that they were laminated. Thereafter, the current collector 7b and the negative electrode 2 were accommodated in the exterior cup 1, while the current collector 7a and the positive electrode 4 were accommodated in the exterior can 3, and the exterior cup 1 and the exterior can 3 were put together. Further, the circumferential portions of the exterior cup 1 and the exterior can 3 were crimped to be tightly sealed with the insulating gasket 6 being interposed therebetween. The button-type secondary battery was prepared in this manner.

Using the prepared button-type secondary battery (battery for evaluation), the battery characteristics were evaluated through the tests described below. The results are shown in Table 1 below.

In the following tests, the process in which lithium ions are adsorbed and stored in the negative electrode material is assumed as charging, and the process in which lithium ions are released from the negative electrode material is assumed as discharging.

Test 1: Discharging Capacity and Initial Charging-Discharging Efficiency (Initial Charging-Discharging Characteristic)

First, constant-current charging was performed at a current value of 0.9 mA until the circuit voltage reached 0 mV. When the circuit voltage reached 0 mV, the constant-current charging was changed to constant-voltage charging, and the charging was continued until the current value reached 20 μA. Based on the amount of current carried during this process, a charging capacity (unit: mAh) was determined. Thereafter, the battery was rested for 120 minutes. Next, constant-current discharging was performed at a current value of 0.9 mA until the circuit voltage reached 1.5 V. Based on the amount of current carried during this process, a discharging capacity (unit: mAh) was determined. These processes were treated as the first cycle.

Based on the charging capacity and the discharging capacity in the first cycle, the initial charging-discharging efficiency (unit: %) was determined according to the following equation. The results are shown in Table 1 below. When this value is larger, the initial charging-discharging characteristic can be rated as more excellent.

Initial ⁢ charging - discharging ⁢ efficiency = ( Discharging ⁢ capacity / Charging ⁢ capacity ) × 100

Test 2: 25° C. Input Resistivity (Input Characteristic)

Constant-current charging was performed at 1.0° C. in a 25° C. temperature atmosphere until the circuit voltage reached 3.82 V. Thereafter, the atmosphere was adjusted to 0° C. temperature atmosphere, and the battery was rested for 3 hours.

Next, charging was performed at 0.5 C for 10 seconds, followed by a rest of 10 minutes; then, discharging was performed at 0.5 C for 10 seconds, followed by a rest of 10 minutes.

Next, charging was performed at 1.0 C for 10 seconds, followed by a rest of 10 minutes; then, discharging was performed at 0.5 C for 20 seconds to have 50% state of charge (SOC), followed by a rest of 10 minutes.

Next, charging was performed at 1.5 C for 10 seconds, followed by a rest of 10 minutes; then, discharging was performed at 0.5 C for 30 seconds to have 50% SOC, followed by a rest of 10 minutes.

Next, charging was performed at 2.0 C for 10 seconds, followed by a rest of 10 minutes; then, discharging was performed at 0.5 C for 40 seconds to have 50% SOC, followed by a rest of 10 minutes.

The discharging capacity (unit: mAh) obtained in Test 1 was multiplied by the respective C rates (0.5 C, 1.0 C, 1.5 C, and 2.0 C) to calculate current values. In addition, a voltage (10-second value) when charging was performed at each of the C rates was determined.

The results at the respective C rates were plotted using the current values as the x coordinates and the voltages as the y coordinates, and the inclination of the straight line of linear approximation thereof was calculated through least square. The inclination was treated as the input resistivity (unit: Ω). When this value is smaller, the input characteristic can be rated as excellent.

Further, a relative value (ratio) of the input resistivity of each example to the input resistivity of Example 1 was determined according to the formula shown below and defined as 25° C. input resistivity. The results are shown in Table 1 below.

25 ⁢ ° ⁢ C . Input ⁢ resistivity = ( Input ⁢ resistivity ⁢ of ⁢ each ⁢ example / Input ⁢ resistivity ⁢ of ⁢ Example ⁢ 1 ) × 100

Test 3: 45° C. Cycle Durability (Cycle Capacity Maintaining Characteristic)

Constant-current charging was performed at 0.5 C in a 45° C. temperature atmosphere until the circuit voltage reached 4.1 V. Thereafter, the battery was rested for 10 minutes.

Next, discharging was performed at 0.5 C until the circuit voltage reached 2.5 V, followed by a rest of 10 minutes.

The charging and the discharging as above were repeated 100 times in total, and the ratio (45° C. cycle durability, unit: %) of the 100th discharging capacity to the initial discharging capacity was calculated.

The results are shown in Table 1 below. When this value is larger, the cycle capacity maintaining characteristic can be rated as excellent.

Example 2

Carbonaceous substance-coated graphite particles were prepared and evaluated in the same manner as in Example 1 except that the amount of the carbonaceous precursor added was changed to such an amount that the content of a carbonaceous substance to be finally obtained would be 5.0 mass %. The results are shown in Table 1 below.

Example 3

Carbonaceous substance-coated graphite particles were prepared and evaluated in the same manner as in Example 1 except that the amount of the carbonaceous precursor added was changed to such an amount that the content of a carbonaceous substance to be finally obtained would be 6.0 mass %. The results are shown in Table 1 below.

Example 4

Carbonaceous substance-coated graphite particles were prepared and evaluated in the same manner as in Example 2 except that the time of performing the intermediate exposure was changed to 10 minutes. The results are shown in Table 1 below.

Example 5

Carbonaceous substance-coated graphite particles were prepared and evaluated in the same manner as in Example 2 except that the first baking temperature was changed to 1,000° C. The results are shown in Table 1 below.

Example 6

Carbonaceous substance-coated graphite particles were prepared and evaluated in the same manner as in Example 1 except that the intermediate exposure temperature was changed to 60° C. The results are shown in Table 1 below.

Comparative Example 1

In Comparative Example 1, the carbonaceous precursor (phenolic resin) was not attached to the graphite particles (spherically-shaped graphite), and only the graphite particles (spherically-shaped graphite) were subjected to baking (first baking, intermediate exposure, and second baking) in the same manner as in Example 5.

Comparative Example 2

Preparation of Graphite Particles

Graphite particles being spherically-shaped graphite were obtained in the same manner as in Example 1.

Preparation of Precursor-Attached Graphite Particles

Precursor-attached graphite particles having the carbonaceous precursor (phenolic resin) attached to surfaces of the graphite particles (spherically-shaped graphite) were obtained in the same manner as in Example 1.

Preparation of Carbonaceous Substance-Coated Graphite Particles

The obtained precursor-attached graphite particles were put in a graphite container having a lid and subjected to baking using a tubular furnace.

Specifically, first, an atmosphere (non-oxidizing atmosphere) where nitrogen was circulated at 2 L/min was prepared, and then the precursor-attached graphite particles were heated at a first baking temperature of 950° C. for 60 minutes.

Thereafter, without cooling (intermediate exposure) and with the atmosphere (non-oxidizing atmosphere) where nitrogen was circulated at 2 L/min being maintained, the particles were heated at a second baking temperature of 1,100° C. for 60 minutes.

Consequently, the carbonaceous precursor (phenolic resin) turned into a carbonaceous substance, thus obtaining carbonaceous substance-coated graphite particles in which surfaces of the graphite particles (spherically-shaped graphite) were coated with the carbonaceous substance.

Physical properties of the obtained carbonaceous substance-coated graphite particles were determined by the foregoing methods. The results are shown in Table 1 below.

Comparative Example 3

Preparation of Graphite Particles

Graphite particles being spherically-shaped graphite were obtained in the same manner as in Example 1.

Preparation of Precursor-Attached Graphite Particles

Precursor-attached graphite particles having the carbonaceous precursor (phenolic resin) attached to surfaces of the graphite particles (spherically-shaped graphite) were obtained in the same manner as in Example 1.

Preparation of Carbonaceous Substance-Coated Graphite Particles

The obtained precursor-attached graphite particles were put in a graphite container having a lid and subjected to baking using a tubular furnace.

Specifically, an atmosphere (non-oxidizing atmosphere) where nitrogen was circulated at 2 L/min was prepared, and then the precursor-attached graphite particles were heated at a baking temperature of 1,100° C. for 60 minutes.

Consequently, the carbonaceous precursor (phenolic resin) turned into a carbonaceous substance, thus obtaining carbonaceous substance-coated graphite particles in which surfaces of the graphite particles (spherically-shaped graphite) were coated with the carbonaceous substance.

Physical properties of the obtained carbonaceous substance-coated graphite particles were determined by the foregoing methods. The results are shown in Table 1 below.

Comparative Example 4

Preparation of Graphite Particles

Graphite particles being spherically-shaped graphite were obtained in the same manner as in Example 1.

Preparation of Precursor-Attached Graphite Particles

Precursor-attached graphite particles having the carbonaceous precursor (phenolic resin) attached to surfaces of the graphite particles (spherically-shaped graphite) were obtained in the same manner as in Example 1 such that the carbonaceous substance content of carbonaceous substance-coated graphite particles to be finally obtained was 2.0 parts by mass.

Preparation of Carbonaceous Substance-Coated Graphite Particles

The obtained precursor-attached graphite particles were put in a graphite container having a lid and subjected to baking using a tubular furnace.

Specifically, the first baking was performed in the same manner as in Example 1.

Next, the intermediate exposure was performed in the same manner as in Example 1 to obtain an intermediate exposure product.

Subsequently, second attachment of the carbonaceous precursor was performed. More specifically, phenolic resin was attached to the intermediate exposure product under the same conditions as those for <<Preparation of Precursor-attached Graphite Particles>> in Example 1. In this process, phenolic resin was attached such that the carbonaceous substance content of carbonaceous substance-coated graphite particles to be finally obtained was 2.0 parts by mass.

Finally, the second baking was performed in the same manner as in Example 1.

Consequently, the carbonaceous precursor (phenolic resin) turned into a carbonaceous substance, thus obtaining carbonaceous substance-coated graphite particles in which surfaces of the graphite particles (spherically-shaped graphite) were coated with the carbonaceous substance. The carbonaceous substance content of the obtained carbonaceous substance-coated graphite particles was 4.0 mass %.

Physical properties of the obtained carbonaceous substance-coated graphite particles were determined by the foregoing methods. The results are shown in Table 1 below.

	TABLE 1
	Example	Comparative Example

	1	2	3	4	5	6	1	2	3	4

Baking	First baking	Atmosphere	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-
			gen	gen	gen	gen	gen	gen	gen	gen	gen	gen
		Temp. [° C.]	950	950	950	950	1000	950	1000	950	1100	950
		Time [min]	60	60	60	60	60	60	60	60	60	60
	Intermediate	Atmosphere	Air	Air	Air	Air	Air	Air	Air	—	—	Air
	exposure	Temp. [° C.]	80	80	80	80	80	60	80	—	—	80
		Time [min]	30	30	30	10	30	30	30	—	—	30

	Second attachment of	—	—	—	—	—	—	—	—	—	Done
	carbonaceous precursor

	Second	Atmosphere	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	Nitro-	—	Nitro-
	baking		gen	gen	gen	gen	gen	gen	gen	gen		gen
		Temp. [° C.]	1100	1100	1100	1100	1100	1100	1100	1100	—	1100
		Time [min]	60	60	60	60	60	60	60	60	—	60

Carbonaceous	Elastic modulus [GPa]	11	18	23	19	19	11	4	9	8	9
substance-	Particle size [μm]	7.0	7.3	7.6	7.5	7.7	7.2	7.0	7.0	7	7.2
coated	Specific surface area [m2/g]	5.0	10.2	14.4	11.0	9.3	5.0	11.0	4.5	4.3	8.0
graphite	Carbonaceous substance	4.0	5.0	6.0	5.0	5.0	4.0	0	4.0	4.0	4.0
particles	content [mass %]
Battery	Discharging capacity [mAh]	13.8	14.3	14.3	14.2	14.3	13.8	14.0	13.7	13.8	13.7
characteristics	Initial charging-discharging	88.1	90.3	92.0	90.0	90.6	88.2	79.5	82.0	82.1	84.1
	efficiency [%]
	(Initial charging-discharging
	characteristic)
	25° C. Input resistivity [%]	100.0	102.1	112.0	104.3	101.8	101.0	121.3	105.0	104.0	103.0
	(Input characteristic)
	45° C. Cycle durability [%]	88.0	90.2	92.0	90.0	90.0	88.3	78.0	84.3	84.5	86.0
	(Cycle capacity maintaining
	characteristic)

Summary of Evaluation Results

As shown in Table 1 above, Examples 1 to 6 with the graphite particles being coated with a carbonaceous substance and the elastic modulus being not less than 10 GPa were excellent in cycle capacity maintaining characteristic compared to Comparative Examples 1 to 4 that do not satisfy those conditions. Furthermore, Examples 1 to 6 were also excellent in initial charging-discharging characteristic and input characteristic.

REFERENCE SIGNS LIST

• • 1: exterior cup
  • 2: negative electrode
  • 3: exterior can
  • 4: positive electrode
  • 5: separator
  • 6: insulating gasket
  • 7a: current collector
  • 7b: current collector